lib/nfs/Connection: fix memory leak (and assertion failure)

nfs_destroy_context() will invoke all pending callbacks with
err==-EINTR.  In CancellableCallback::Callback(), this will invoke
NfsConnection::DeferClose(), which however is only designed to be
called from nfs_service().  In non-debug mode, this will leak memory
because nfs_close_async() is never called.

Workaround: before nfs_destroy_context(), invoke nfs_close_async() on
all pending file handles.

src/lib/nfs/Connection.cxx

@@ -132,6 +132,17 @@ NfsConnection::CancellableCallback::CancelAndScheduleClose(struct nfsfh *fh)
 	Cancel();
 }
 
+inline void
+NfsConnection::CancellableCallback::PrepareDestroyContext()
+{
+	assert(IsCancelled());
+
+	if (close_fh != nullptr) {
+		connection.InternalClose(close_fh);
+		close_fh = nullptr;
+	}
+}
+
 inline void
 NfsConnection::CancellableCallback::Callback(int err, void *data)
 {
@@ -370,6 +381,10 @@ NfsConnection::DestroyContext()
 	if (SocketMonitor::IsDefined())
 		SocketMonitor::Cancel();
 
+	callbacks.ForEach([](CancellableCallback &c){
+			c.PrepareDestroyContext();
+		});
+
 	nfs_destroy_context(context);
 	context = nullptr;
 }